The permeability of single arterial, mid and venous capillaries in the frog mesentery to electrolytes such as K ion will be determined under known conditions of flow, geometry and extravascular interaction. The basic method for study involves the application of the indicator diffusion principle. This is possible at the capillary level due to the advent of K ion selective ion exchanger microelectrodes. The indicator curves are recorded at an upstream and a downstream site. The difference between the two curves is determined by extraction. Permeability is calculated from a knowledge of flow, surface area and extraction.